Carcinogenesis is accompanied by increased production of oxygen free radicals (OFRs). OFRs degrade membranes by lipid peroxidation, releasing volatile organic compounds (VOCs) which are excreted in the breath. Hence, breath VOC analysis may detect biomarkers of cancer. Breath VOCs were analyzed in 108 patients with abnormal chest x-rays. 60 were found to have lung cancer at bronchoscopy. 36 VOCs, predominantly methylated alkanes, were significantly different in the lung cancer patients. A discriminant analysis model (with 20 VOCs) was 90% sensitive and 87.5% specific for lung cancer. Hypothesis: Breath VOC analysis may provide a clinically useful new test for early diagnosis of lung cancer. Experimental plan to test the hypothesis: 1. Breath VOCs will be analyzed in 200 patients undergoing bronchoscopy because of an abnormal chest x-ray suggestive of lung cancer. 2. The probability of lung cancer will be predicted from the breath test. 3. Breath test predictions will be compared to the biopsy diagnosis, and the sensitivity, specificity, and predictive value of the breath test will be determined. The long-term aim of the research is to determine the diagnostic value of breath testing as a tool for the early detection of lung cancer in mass screening surveys. Proposed commercial applications: Breath testing could provide a non-invasive diagnostic tool for the early detection of lung cancer in mass screening surveys.